The present invention relates to electrostatic spray guns, and more particularly to an apparatus for indicating when such spray guns are operating effectively.
Electrostatic spray guns have been used for many years in the field of spray painting and spraying and atomizing other types of charged particles for depositing upon conductive articles. In the field of spray painting it has become more or less conventional to place the article to be coated at an electrical ground voltage potentional, and to develop an electrostatic field by means of an electrode placed in the proximity of the spray gun itself, the field being developed between the electrode and the grounded article. Further, it has become generally acceptable in the industry that the best electrostatic spraying efficiencies are obtained when the charging electrode on the spray gun itself is in reasonably close proximity to the discharge orifice where particles are emitted from the spray gun. Such electrodes usually take the form of sharply pointed needles or edges, with the understanding that the elctrostatic field generated from a point of sharp discontinuity creates the highest electrostatic field intensity in the region of the sharp discontinuity. When this discontinuity is placed in the region near the paint orifice, the emitted paint particles are then subject to the maximum electrostatic field intensity, and the paint particles are most effectively voltage charged. Electrostatic paint spraying is usually accomplished in combination with other forms of paint atomization, as for example, atomization by means of impinging air jets upon an emitted column of paint, or hydraulic atomization wherein high hydraulic paint pressures force the liquid paint through a very constricted orifice. In these cases the electrostatic field developed between a point electrode and a grounded article causes the atomized paint droplets to become voltage charged, and thereby enhances the attractive forces developed between the paint droplets and the grounded article to be coated.
Since the electrostatic field developed between a spray gun and a grounded article is invisible, it is sometimes difficult to determine whether and to what extend the electrostatic field is contributing to painting efficiency. Of course, the electrostatic field developed at a spray gun can be detected by an operator in bringing the electrodes in close proximity to his body, but this is not only discomforting, it can also be hazardous. An experienced operator can detect when an electrostatic voltage potential has disappeared or diminished by observing the quality of coating on the article itself, but this is difficult to detect and may result in a number of articles being inefficiently coated before the operator notices a change in coating efficiency. An inexperienced operator may very well not realize any change in coating efficiency as a result of a drop or loss of electrostatic voltage and could very well operate an electrostatic spray gun for a long period of time without realizing that the electrostatic voltage is not being generated by the gun.
Automatic electrostatic paint spraying systems are gaining wider acceptance in the industry, wherein a plurality of electrostatic spray guns are mounted on a fixed or movable frame assembly, alongside a moving conveyor line upon which articles are mounted. These spray guns are frequently mounted inside of large spray painting booths, and a plurality of such guns may be monitored by a single operator. If electrosatic voltage fails on any one of these spray guns it may be difficult or impossible for an operator to detect any change in the overall operation of the system, where the performance of a single spray gun is difficult to evaluate in a context of a large system of a number of guns operating simultaneously. Voltage indicators have been incorporated into high voltage power supplies for the purpose of providing such a monitoring mechanism, but such indicators only provide a verification that high voltage is being developed at the power supply. If a break in the voltage circuit between the power supply and the electrode occurs, it may be very difficult for the operator to detect that system performance is degraded.
Therefore there is a need to provide a simple and effective indicating mechanism which provides an indication of the operating effectiveness of a high voltage circuit in a spray gun. Further, there is a need to provide such a mechanism which is responsive to the electrostatic field generated at the electrode itself, rather than being responsive to other circuits which do not include the entire voltage path contained in a high voltage circuit.